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And you are correct , most installers do not concern themselves with introduced air during a changeout.

There should be auto air bleeders at the highest point in the system , but those are un-reliable in my experience , you can :

- remove the air bleeders from the highest point , which is easy if there are shut off valves , you can do even if valves are not present b/c the water pressure is usually the lowest at that point , do it hot and install a ball valve , then check for air by opening the valve. when finished replace the bleeder.

- a better option is , if an air seperator is present , they are far quicker and more effective at removing air from the loop , a nice 1/2" ball valve on the top of that device will rid air in a heartbeat.

Check your makeup water , is it on? , is the regulator adjusted correctly? You will have to add water if you get much air out , if there is an expansion tank with a site glass , I remove air over and over and over at the serarator while throttling make up water at the same time , eventually you will see a bit of water in the site glass on the expansion tank. A good make up water system will have a regulator bypass , that makes the process faster.

Your system may not have these components or different versions , you may have to get creative , but a proper system should have these components in one variation or another.

Sorry for the jumbling on the info.

Your water level is most likely low , have any units been shutting off on HPS?